We have a craft swap thread somewhere on this thread. The sender pays the shipping, and the giver promises to pay it forward in the future. It didn't go very far, unfortunately. I did receive some wonderful knitting looms through it - I have thoroughly enjoyed them. In addition, the same thing has gone on over on FB - the giver pays the shipping, knowing that sometime in the future, she might receive something in return, and also the good feeling that her stuff has found a nice home.
It really has to be done on that kind of system - the giver pays for shipping, knowing that her stuff is finding a home where it will be used, and thinking, "Well, maybe I'll get something in return from someone else."
I don't think asking for shipping money would work very well. Someone might get rooked out of their money, and feel cheated.